Strain gauges



Feb. 21, 1967 Tosmo DO! STRAIN GAUGES Filed July 9, 1964 INVENTQR.Taslu'o )of w rs E Mwl m United States Patent 3,305,815 STRAIN GAUGESToshio Doi, Minamitama-gun, Tokyo-t0, Japan, assignor t0 KabushikiKaisha Hitachi Seisakusho, Tokyo-to, Japan, a joint-stock company FiledJuly 9, 1964, Scr. No. 381,438 Claims priority, application Japan, July13, 1963, 38/ 38,299 2 Claims. (Cl. 338-2) This invention relates tostrain gauges of the type wherein electrical resistance variations in agauge filament due to strain is utilized to measure the strain. Moreparticularly the invention concerns new strain gauges of thi type madeof binary alloys of platinum and osmium and ternary alloys of platinum,rhodium, and osmium.

Generally stated, it is an object of the present invention to providestrain gauges of this type having high gauge factors, various otherhighly desirable characteristics, and high uniformity of characteristicsof the product.

Strain gauges of the type with which this invention is concerned are notonly widely used for strain and deformation measurements, in general,but in recent years have also been used as elements to convertmechanical quantities into electrical quantities in industrialinstruments such as self-balancing recorders,

In most applications, such strain gauges consist of fine metal wire ormetal foil arranged, for example, in the form of a grid, which isintimately and firmly secured to an object whose strain is to bemeasured.

The sensitivity of such a gauge is determined by its socalled gaugefactor GF, which is expressed by the following equation.

GF=(1+21/)+(Ap/p)/(Al/l) where:

1/ is Poissons ratio of the gauge filament material; p is the electricalresistivity of the gauge filament material;

and l is the length of the gauge filament.

Since Poissons ratio 1 is approximately 0.3 in the Y case of metals, thequantity represented by (A (-Al/l) must be made large in order to obtaina high value of GP.

The principal requirements for a strain gauge of this type are asfollows: a high value of gauge factor, no variation with temperature ofthe gauge factor, and linearity of its strain-resistance characteristic.Some other requirements are: a high resistivity, a low temperaturecoefficient of resistance, a high elastic limit, good workability,uniform quality with little change with time (long operating life), lowhysteresis, low thermoelectromotive force with respect to copper in thecase of use with direct current, good workability for working into finewire, good mechanical properties, good properties for soldering andbrazing, and high resistance against oxidation and corrosion.

As conducive to a full appreciation of the utility of the 3,305,815Patented Feb. 21, 1967 present invention, the following consideration ofknown strain gauge materials is presented, reference being made to theaccompanying Table 1 in which there are shown results of measurements ofgauge factor, temperature coeflicient of resistance, and electricalresistivity of known strain gauge materials.

TABLE 1 Temp. Coeif. of Resistance, Cf( 10-/ C.)

Electrical Resistivity Q-cm.)

Gauge Factor Material (GF) Cu-Ni i20 Isa-Elastic Ni-Cr :I:20

Manganine M onel Pt Of the materials shown in Table 1, the most widelyused materials are the copper-nickel alloys. These alloys have theadvantages of substantially low temperature coefficient of resistanceand high resistivity. However, their gauge factors are of the order of2, and, because of their nature, exhibit deviations in performance offrom 5 to 10 percent among products in the same lot or between differentlots. Furthermore, since these are alloys of base metals, they have thedisadvantage of low resistance against oxidation and corrosion.

In addition, noble metal alloys are also being used, but none, as far asI am aware, has succeeded in satisfying all of the aforementionedrequirements.

It is an object of the present invention to provide strain gaugematerials which, to a substantial extent, fulfill the aforestatedrequirements.

More specifically stated, it is an object to provide an electricalresistance type strain gauge characterized by its composition comprisinga binary alloy of platinum and osmium or a ternary alloy of platinum,rhodium, and osmium, and having highly desirable and advantageousfeatures including a gauge factor of 4 or higher, relatively lowtemperature coeflicient of resistance, relatively high electricalresistivity, other superior features, and, moreover, high uniformity ofproducts.

The specific nature and details of the invention will be more clearlyapparent by reference to the accompanying drawing and followingdescription with respect to preferred embodiments of the invention,reference being made to Table 2, in which measured values of tensilestrength, resistivity, thermoelectromotive force relative to copper,temperature coefficient or resistance, and gauge factor are shown forexamples of alloys suitable for use in the strain gauge according tothis invention.

The figure in the accompanying drawing shows an example of a straingauge according to this invention, in which a thin metal Wire is used.This example relates to an adherent type single gauge of lattice form.This gauge comprises a gauge base 1 onsisting of an insulating flexiblefilm such as paper, paper impregnated with Bakelite, polyester sheet,etc., a gauge filament 2 adhered onto said base 1, and lead-wires 3.

TABLE 2 Composition Thcrmo- Temp. (atomic percent) Tensile ElectricalElectromotive Coeff. Gauge Strength Resistivity Force With Resist. ClFactor (kg/mm!) (ail-cm.) Respect to (10 0.) (GF) Pt Rh Os Copper (av-l9s 2 83. 0 14.4 +0. 1 24. s 4. 5 95 0 5 114.0 20.1 +5.2 15.1 5.0 90 0176. 0 31. 5 +7. 8 9. 8 5. 5 53 45 2 152. 0 18.8 1. 7 12. 5 4. 2 50 45 5187. 0 21. 4 0. a 10. 4 4. 4 45 45 10 0 2s. 4 +0. 5 7. 3 4. 7 75.2 230.8 70.9 21.1 -3.5 15.5 4.1 55.5 34 0.4 83.1 20. 0 2. 7 14. 2 4.1 87.711 1.3 5&2 20.7 -5.5 25.2 4.1 74.1 0.9 75.5 21.9 -3.8 17.0 4.1 79.3 200.7 68.3 21.1 3.0 15.4 4.1 70 20 1. 0 150. 3 29. 5 +3. 6 7. 9 5. 0

As is observable in Table 2, the gauge factors of all TABLE 3 alloysexceed 4, that in one case being as high as 5.5. The tensile strengthsare considerably high, being in the range Composition (atomic ofapproximately from 60 to 190 kg./mm. At the same p g p p- 0 time, allalloys exhibit good workability and are readily tii fgfg fi workableinto fine wire. Although the electrical resis- Pt Rh 0S tivities, whichare substantially from 20 to 30 microohm/ cm., are somewhat lower thanthat of Cu-Ni alloys, this 328 inadequacy can be overcome by working thewire mate- 90 0 10 3 981 rial to finer diameters. g 328 It is furtherobservable that most of the values of ther- 1 730 mo-electromotive forcewith respect to copper are within 2 45 45 10 3 750 :2 microvolt/deg. C.,and the values of temperature co- 4 730 efficient of resistance are inmany cases near 1.5 10- 5 730 deg. C. or therebelow. If the temperaturecoefficient of resistance of a strain gauge material is in theneighbourhood of 1.5x 10- deg. C., the temperature coefficient ofresistance of the entire system can be readily made small through theuse of a suitable compensation circuit. Furthermore, by using a suitableelastic material (thermal expansion coefficient a corresponding to thethermal expansion coefiicient 54 temperature coefiicient of resistanceCf, and gauge factor GF of the present gauge, it is also possible toobtain the following relationship.

Furthermore, since the alloys used in the strain gauge according to theinvention are noble metal alloys, they have high resistance againstoxidation and corrosion and are readily soldered and brazed. Inaddition, if care is exercised in their heat treatment, these alloys canbe caused to have amply low rates of change with time (amp- Thus, thepresent invention provides a resistance strain gauge withcharacteristics which are superior to those of strain gauges heretoforeavailable. The principle advantageous features of the strain gauge ofthis invention are as follows:

(1) A high gauge factor can be obtained;

(2) The temperature coefiicient of resistance is relatively small;

(3) The deviations in characteristics determining quality of the productwithin one production lot or between lots are extremely small comparedwith those occurring in conventional strain gauges, and accordingly, themanufacturing procedure is simplified;

(4) Resistance to oxidation and corrosion is excellent;

(5) The gauge elements can be easily soldered and brazed;

(6) The gauge material has good workability, whereby they can be readilyworked into fine wire or thin foil;

(7) The resistivity is of a suitable value;

(8) The thermo-electromotive force with respect to copper is relativelysmall; and

(9) The tensile strength is substantially high, and, accordingly, theproportional limit is high, whereby relatively large strains can bemeasured.

Furthermore, the strain gauge of this invention possesses other goodcharacteristics required for strain gauges and is applicable to a widerange of uses.

It should be understood, of course, that foregoing disclosure relates toonly illustrative examples of the invention and it is intended to coverall changes and modifications of details as illustrated by the examplesof the invention herein chosen for the purposes of the disclosure, whichdo not constitute departures from the spirit .and scope of the inventionas set forth in the appended claims.

I claim:

1. In a strain gauge device consisting of a strain element, leads, and abacking strip, an improvement wherein a binary alloy of platinum andosmium containing therein 90 to 99 atomic percent platinum is used assaid strain element.

2. In a strain gauge device consisting of a strain element, leads, and abacking strip, an improvement wherein a ternary alloy of platinum,rhodium and osmium is used as said strain element, said alloy beingcomposed of to atomic percent platinum, 10 to 45 atomic percent rhodium,and 0.1 to 10 atomic percent osmium.

References Cited by the Examiner UNITED STATES PATENTS 1,990,277 2/1935Feussner. 2,206,616 7/1940 DevereuX. 2,991,671 7/1961 Bonn et al 3382 X3,060,395 10/1962 Sandven 3382 3,079,575 2/1963 Singdale et a1 338-23,202,951 8/1965 Krinsky 3382 FOREIGN PATENTS 29,723 4/1911 GreatBritain.

RICHARD M. WOOD, Primary Examiner.

W. D. BROOKS, Assistant Examiner.

2. IN A STRAIN GAUGE DEVICE, CONSISTING OF A STRAIN ELEMENT, LEADS, ANDA BACKING STRIP, AN IMPROVEMENT WHEREIN A TERNARY ALLOY OF PLATINUM,RHODIUM AND OSMIUM IS USED AS SAID STRAIN ELEMENT, SAID ALLOY BEINGCOMPOSED OF 45 TO 90 ATOMIC PERCENT PLATINUM, 10 TO 45 ATOMIC PERCENTRHODIUM, AND 0.1 TO 10 ATOMIC PERCENT OSMIUM.